Controlled selective gene expression plays the key role in embryonic development and cellular differentiation. We plan to study the control of gene expression in embryonal carcinoma cells (ECC) induced to differentiate into entodermal cells either spontaneously or by retinoic acid. Extensive homologies between normal embryonic development and in vitro differentiation of ECC warrant the use of the latter as the valid and more accessible experimental model. To analyze the existence and the extent of post-transcriptional control of gene expression we will compare the sequence complexities of hnRNA and mRNA from both ECC and entodermal cells by molecular hybridization. In the same time we will clone representative mRNAs specific for ECC and entodermal cells using recombinant DNA technology. These specific and abundant probes will be used to study the role of transcription, post-transcriptional events and mRNA stability in control of gene expression. The results obtained should provide a basic framework for analysis of the control of gene expression during normal embryogenesis.